"On my regular toilet I've replaced the 'bad' wax gaskets a few times and my toilet still leaks!!??"
Generally wax doesn't "go bad" on a standard floor-mounted toilet, as its function is not to prevent leaks. Wax on a floor-mounted toilet is there to prevent odors. If you are experiencing a leak coming from underneath the floor-mounted toilet, you probably have a partial (or full blown) toilet stoppage down the drain line (or the toilet is cracked in the bowl). Also, make sure the leak isn't coming from above and dribbling down the back of the toilet, as this is a "typical" undetected problem. If you aren't sure where the leak is coming, from try putting a few drops of food coloring in the tank and waiting a few hours.

"I've seen expensive, sliding type of flanges/gaskets that attach to the bottom of a toilet and then slide into the drain. What's your opinion of them?"
We don't wish to offer them (although we certainly would make more profit doing so) because: 1) They are much more complicated than a simple wax or sponge ring. 2) The toilet bowl must be totally. perfectly clean and spotless. That is never the case with a used toilet and even with a new toilet a lot of times it will have some grease or dirt on them. The toilet bowl must be cleaned very well. 3) The water closet flange, by design, tends to be more susceptible to damage during construction and the basic wax, wax with gasket and sponge take up most problem flanges while many of the "new" type (read "expensive") toilet to flange connectors need to generally have a "perfect" flange and a "perfect" toilet bowl. 4) If a toilet flange is leaking the problem needs to be addressed. That problem is not the wax or sponge. We see it as a case of "if it ain't broke, don't fix it" and so, we prefer to pass on offering these "new" type of connectors. We strive for total customer satisfaction and our fear is that installing these new connectors will produce a lot of frustration.

"Can't I just use a wax ring for my wall-hung toilet?"
Some people do. We absolutely do NOT recommend it because wax does not have "memory." Over time all walls will flex (somewhat) and when that happens, gravity will allow a space - which will allow leaks. We feel that wax gaskets are great for regular toilets but not for wall-hung toilets.